Sessions support

Memcached provides a custom session handler that can be used to store
user sessions in memcache. A completely separate memcached instance is used
for that internally, so you can use a different server pool if necessary. The
session keys are stored under the prefix
memc.sess.key., so be aware of this if you use the
same server pool for sessions and generic caching.

If you are setting data to the session and it immediately disappears and you aren't getting any warnings in your PHP error log, it's probably because your sessions expired sometime in the 1970s.

Somewhere between memcached 1.0.2 and 2.1.0, the memcached session handler became sensitive to the 30-day TTL gotcha (aka "transparent failover"). If your session.gc_maxlifetime is greater than 2592000 (30 days), the value is treated as a unix timestamp instead of a relative seconds count.

This issue is likely to impact anyone with long-running sessions who is upgrading from Ubuntu 12.04 to 14.04.

While the previous poster has a point that Memcached can and will cleanup to make room for it's keys, the likelihood of active sessions (due to the likelihood that they will be written to again within 30 seconds) is fairly low provided you have your memory allocation properly alloted.